Filtration modules may be used for the filtration of fluids and are known in a wide variety of variants. A filtration module usually comprises a housing with one or more filter elements, e.g. tubular membranes as shown in U.S. Pat. No. 5,494,577, positioned therein which divide the space inside the housing in a feeding side and permeate side.
One type of filtration module is based upon so-called dead-end filtration in which fluid to be filtered is fed to the feeding side, from there is passed through the filter element(s), wherein the solids are trapped in the filter and permeate (filtered fluid) is released at the permeate side.
As the filter elements get fouled over time, counter measures are required at least once in a while. A common countermeasure is to regularly provide a backwash flux through a filtration module to remove the solids from the filter and abduct the solids from the filtration module/filtration system. Backwashing is typically done by reversing flow directions of feed and permeate flows.
In typical high-volume industrial applications it is common to arrange multiple filtration modules in an array and connect them in a modular way to provide the required filtration capacity. To connect the filtration modules, use is made of so-called headers and/or end caps. The headers and end caps are configured to provide a flow path to the feeding side of each filtration module and to provide a flow path from the permeate side of each filtration module. An example of such a modular arrangement, wherein use is made of headers/end caps, is shown in U.S. Pat. No. 5,405,528.
When designing a filtration system with multiple filtration modules, there are two main design criteria that are preferably fulfilled. The first criterion is to minimize the size of the footprint of the filtration system. The second criterion is to minimize the standard deviation or variance of the performance of the multiple filtration modules. The first and second criteria may interfere with each other, so that a compromise needs to be sought.
The first criterion tries to maximize the number of filtration modules per unit of area, so that the maximum capacity can be obtained with the least occupied space.
The second criterion relates to the global settings of a filtration system. In order to protect the filtration modules from excessive fouling the global settings have to be based on the fouling rate of the worst performing modules. The worst performing modules are the modules with the highest filtration flux and/or the lowest backwash flux.
Therefore, it is desirable that each module performs as close to the average as possible, which means that a filtration system design should aim to minimize the pressure difference between the modules.
Traditional designs comprise a header with many side branches, wherein a filtration module is connected to a side branch via an end cap. In this design, the pressure difference between two filtration modules is determined only by the header. Depending on the number of filtration modules and the expected flow rate, the header size may be chosen to achieve an acceptable pressure gradient. Although the end caps cause hydraulic pressure losses, they do not contribute to the pressure difference between filtration modules, because the flow path through each filtration module comprises the same number of end caps. Hence, this design performs well in terms of the second criterion. However, from a first criterion point of view, the design can still be improved.
In an alternative design, with the aim to reduce the size of the footprint of the filtration system, the headers and the end caps are integrated as shown for instance in U.S. Pat. No. 5,405,528. Such a design may have advantages such as a smaller footprint and fewer parts/less material usage. However, in such a design the dimensions can not be adapted to the number of filtration modules or expected flow rates, especially not when the end caps are standardized in size. A disadvantage of the alternative design is thus that the friction in the end cap does contribute to the pressure difference between filtration modules, because the flow path for each filtration module comprises a different number of end caps.